Planar waveguide device alignment is typically provided by an external precision positioning system such as multi-axis translation/rotation stages driven by piezo-electric micropositioners, for example. An attachment material such as an organic adhesive or metallic solder is generally positioned between aligned substrates and used to maintain long-term substrate alignment. While precision positioning systems can generally align waveguide arrays on two planar waveguide devices to within less than 0.2 μm lateral misalignment, a critical issue is maintaining waveguide alignment during adhesive curing without high rates of post-attachment rework.
In laser welding applications, post-weld bending rework is often required to achieve sub-micron lateral alignment of single waveguides. Using this method to align arrays of planar waveguides is more difficult when the bending rotation occurs in the plane parallel to the planar substrate: the rotation introduces a gap at one end of the waveguide array, introducing unwanted axial misalignments and coupling losses.
Crystal block attachment, such as disclosed in U.S. Pat. No. 6,512,642 “Method and structure for aligning optical elements”, assigned to the owner of the present application, is one solution for zero-shift attachment of devices to substrates, but the cantilever nature of the planar device attachment to the common alignment substrate via an intermediate block makes the approach less suitable for planar devices requiring wirebonded electrical interconnections. Additionally, a full six-axis positioning system is required to align the two substrates.
An alternative planar device alignment and attachment approach involves microactuators that are integrated into or mounted on the common alignment substrate. For example, integrated micro actuators have been previously described that are based on expansion and/or contraction of piezoelectric materials, electro-strictive materials, magneto-strictive materials and magnetic materials. Microactuators have also been fabricated based on electrostatic forces between plates and substrates, electrically-induced shape changes in polymers and ultrasonic excitation of flexure elements. Another common integrated microactuator approach takes advantage of material expansion and/or contraction via the thermo-mechanical effect. These microactuator structures, which are often integrated into planar substrates using MEMS and semiconductor fabrication processes, can be broadly categorized into two groups: (1) Microactuators based on heating of selected supporting arms or beams, where the arms or beams are fabricated using a single material. Since remaining arms, beams or supporting structures are unheated, CTE-induced differential forces on the arms or beams lead to deformation (translation and/or rotation) of the microactuator structure; (2) Microactuators that deform because supporting arms are bi-material laminates fabricated from materials with two different CTEs. When these arms are heated, they provide actuation by curving toward the lower-CTE material.
A disadvantage with all of these actuation approaches is that they require continuous control input (e.g., an electrical signal) in order to maintain their position. This is a significant disadvantage in microalignment of hybrid optical components, since it means this control input must be maintained over the life of the product. Further, many of the microalignment technologies described above require complex precision microstructures (MEMS, for example) that increase product cost. MEMS actuators can be designed with integrated latch structures so that their position is maintained after the control input is removed. Yet these actuators can be expected to be even more complex in design than standard MEMS mentioned, resulting in even higher product cost.
Another type of thermo-mechanical actuator can be formed by heating a polymer material in an enclosed cavity so that when it expands a force is exerted in a predefined direction. This actuator solution is relatively simple compared to MEMS actuator approaches, but constant application of control input (e.g., electric potential for a polymer heater) is still required. Also, since actuation is provided by thermo-mechanical polymer expansion, changes in ambient temperature could induce changes in polymer size, resulting in unwanted shifts in actuator position.
A family of IR-absorbing Pyrex™-like glasses was recently developed at Corning. This process enables dark glass bump formation for bump heights of up to 70-100 μm. These bumps can be used as standoff structures for alignment of planar waveguide device substrate. One or more laser heating cycles direct energy to the bump to increase its height. Using a closed-loop control system, the bump height can be slowly increased until a target height is reached.